1. Field of the Invention
This invention pertains generally to sedimentation equipment for use in applications such as water purification, sewage treatment and liquid waste clarification. In particular, this invention comprises a novel structure for raking sediment at the bottom of a settling tank.
2. State of the Art
A settling tank is usually of cylindrical configuration, and is commonly provided with a rotary rake structure to move sediment along the bottom of the tank. In general, the transverse diametric dimension of a settling tank is several times larger than the depth. A rotary rake structure for use in a settling tank ordinarily comprises a torque transmitting member mounted for rotation about the axis of the tank and a pair of rake arms attached to the torque transmitting member. The rake arms extend radially outward in opposite directions from the torque transmitting member, and support scraping devices for moving sediment on the bottom of the tank to a well that is usually located at the center of the tank. A settling tank provided with a rotary rake structure of a kind known to the prior art is described in U.S. Pat. No. 3,770,131, which is assigned to Envirotech Corporation.
Typically, the torque transmitting member of a rotary rake structure comprises either a cage structure or a vertical shaft, and each of the two oppositely extending rake arms is a frame structure comprising one or more upper chords and one or more lower chords. The configurations of the two rake arms are substantially the same for a given rotary rake structure. The upper and lower chords of each rake arm are fixedly positioned with respect to each other by truss members, and each rake arm is attached to the torque transmitting member in such a way as to maintain a desired orientation with respect to the bottom of the settling tank. Scraping devices depend from the lower chords of the rake arms to sweep along the bottom of the settling tank when torque is applied to the rake arms by the torque transmitting member.
According to design considerations prevailing in the prior art, the lower chords of the rake arms were attached to the torque transmitting member in such a way as to accept the transmission of all the torque applied to the rake arms by rotation of the torque transmitting member. Thus, the joint connecting one of the lower chords to the torque transmitting member had to be able to withstand very high shearing force components in both the horizontal and vertical directions.
Furthermore, according to the prevailing design considerations of the prior art, the trusses holding the upper chords in fixed position relative to the lower chords accepted the vertical force components attributable to the weight of the rake arms and the weight of any solids that settle on the frame structures of the rake arms. The upper chords in the prior art were merely braced for stabilizing the other members of the rake arm structure, and were not considered to be subject to vertical shearing forces. In the prior art, the loads accepted by the upper chords were considered to be essentially horizontally directed along the axis of the upper chords.
In the prior art, the torque transmitting member extended downward all the way to the lower chords of the rake arms. The width of that portion of the torque transmitting member between the upper and lower chords of the rake arms usually imposed a limitation on the width of the rake arms because attached of a rake arm to the torque transmitting member was most practically accomplished by aligning the upper and lower chords of the rake arm with corners on the torque transmitting member.
Where the torque transmitting member in the prior art was a cage structure, the lateral dimensions of the cage structure had to be greater than necessary for the application of torque to the rake arms. The extra width of the cage structure in the prior art was necessary to provide an unobstructed sediment well and/or feed inlet arrangement adjacent the axis of the torque transmitting member. In the prior art, the bottom portion of the cage structure extended close to the bottom of the tank. If the downward extension of the cage structure could have been lessened (i.e., if the bottom portion of the torque transmitting member could have been raised significantly above the floor of the tank), a cage structure of smaller width (and hence of less weight) could have been used.
In installing rake arms in the prior art, each rake arm was individually positioned so that its lower chord (or chords) would assume an appropriate orientation with respect to the contour of the floor of the tank. The orientation of the lower chords had to be such that scraping devices affixed to the lower chords could effectively sweep along the floor of the tank during rotation of the rake arms. Adjustment of the orientation of each rake arm was accomplished by means of a turnbuckle or threaded rod "take-up" connection at the upper chords, or by means of shims placed between attachment flanges on the lower chords. The rake arms of the prior art, after being individually positioned, were then permanently secured to the torque transmitting member (as by welding or bolting) to form a single rigid structure extending diametrically across the settling tank.
The process of welding or otherwise attaching each of the upper and lower chords of a rake arm to the torque transmitting member in the prior art had to be performed with precision in order to maintain the proper orientation of the rake arm. For a typical two-arm rotary structure of the prior art, about 8 separate upper and lower chords and truss members had to be welded or bolted to the torque transmitting member. A lack of precision in the forming of any one of these joints could destroy the proper orientation of a rake arm.